It can be useful to know if the alpha plane consists of fully opaque
pixels or not, in which case it can e.g. safely be stripped.
This only requires a very minor modification to the AVX2 routines, adding
an extra AND on the read alpha value with the reference alpha value, and a
single extra cheap test per line.
detect_alpha_8_full_c: 2849.1 ( 1.00x)
detect_alpha_8_full_avx2: 260.3 (10.95x)
detect_alpha_8_full_avx512icl: 130.2 (21.87x)
detect_alpha_8_limited_c: 8349.2 ( 1.00x)
detect_alpha_8_limited_avx2: 756.6 (11.04x)
detect_alpha_8_limited_avx512icl: 364.2 (22.93x)
detect_alpha_16_full_c: 1652.8 ( 1.00x)
detect_alpha_16_full_avx2: 236.5 ( 6.99x)
detect_alpha_16_full_avx512icl: 134.6 (12.28x)
detect_alpha_16_limited_c: 5263.1 ( 1.00x)
detect_alpha_16_limited_avx2: 797.4 ( 6.60x)
detect_alpha_16_limited_avx512icl: 400.3 (13.15x)
9b8b78a815